Playable Borg is a very inviting concept, however, I highly doubt it will ever happen. We may see the odd playable ship (would prove extremely popular and people would pay handsomely). If it does ever happen, it'll more likely be the Borg Cooperative.
*Yes I know, this thread is old, but it came up upon searching*
Much of the previous argument against the Borg as a faction race depends on the "individual" component. As a federation officer, I am given orders to follow. How I follow them leads to either success or failure. The illusion of choice may be given in some circumstances but the result "mostly" remains the same. There are some occasions these days that give choices, but many of the core missions once upon a time had 2 major outcomes.
As a Borg race, it could be very much the same. Breaking canon means a complete overhaul of the species verses a playable faction.
One could argue that they wouldn't be warlike and engage other species unless deemed a threat or for assimilation. However the Borg are actively engaged in war with the races at present; that solves that issue.
The story direction could follow along a line of civil war; a true Borg faction, seeking the re-assimilation or obliteration of all entities formerly of the Collective. I liked the one suggestion, that the player would represent a voice of the Collective. A voice representing a Tactical, Technologist (engineering), or Scientific focus of the Collective.
As a Borg purist I would love to play this faction; to experience "their" side of the story.
I can understand, on a strict sense, for the sake of universe, that watching a true Borg sprint around an event to gain trinkets to unlock a Breen vessel would look ridiculous. Yes it could be explained. (Q can do anything he really wants) but from that same perspective, would a Vulcan do that? A Klingon? We can weigh down opportunity in the name of universe, but like every other MMO I've played, the immersion is broken for the sake of playability and fun.
I learned to let go of that issue a long time ago. Just because I will take it as a serious role to play as a Borg, doesn't mean everyone else will; and why should they be required too? It is, in fact, a game.
I confess, I'm behind in this game. Started years ago, missed a lot of changes, additions. The Romulan faction was a wonderful addition. But I'm still looking for an Armada fix; to be the Borg, to fly a Cube, to assimilate worlds. I know I'm not the only one out there.